I. FIELD OF THE INVENTION
This invention relates generally to inserts for rolling cone rock bits and more particularly to conical type inserts having specially shaped cutting tips.
II. DESCRIPTION OF THE PRIOR ART
Rock bits using sintered tungsten carbide inserts with cutting tips having a generally wedge or chisel-shaped configuration have been used for drilling soft and medium formations. Various configurations of wedge-shaped inserts are shown in U.S. Pat. No. 3,442,342. Inserts of this type usually have a pair of cylindrical flanks that converge into an elongated rounded crest.
U.S. Pat. No. 4,018,260 shows a modified version of the chisel-shaped inserts in which the leading flank is concave and the trailing flank is convex. The scoop-shaped leading flank aids in lifting cuttings while the convex trailing flank resists breakage because of the additional support provided.
A shortcoming with the scooped inserts of the '260 patent is that the flank surfaces are flat and intersect the side conical surfaces at sharp angles, thereby creating stress risers which promote breakage.
Recently, conical inserts have been utilized in rock bits for drilling soft, medium and hard formations. Conical inserts include a cylindrical base and a cutting tip comprising a conical surface concentric with the insert axis converging into a spherically shaped apex.
U.S. Pat. No. 4,334,586 illustrates a plurality of conical type inserts in which the spherical crowns are asymmetrical with respect to the insert axes. On these inserts, the side surface of the cutting tip below the crown includes straight line surfaces between the base of the cutting tip and the crown to form an oblique cone structure. (See FIGS. 3 and 4 of the above mentioned patent.)
U.S. Pat. No. 4,776,413 teaches the use of a conical type cutter having a spherical apex with the side surface between the base of the cutting tip and the apex being a concave surface of revolution.
Another conical type insert commonly used is an "ogive" insert having a symmetrical convex surface forming the cutting tip. The problem with such an insert is that the extra material forming the convex surface caused less penetration and lower rates of penetration. Such inserts are typically used for hard formations only.